Wet scrubbers are a simple and commonly used approach to removing contaminants from gas streams. The principle of a wet scrubber is to remove contaminants from the gas stream by passing the stream through a reaction chamber, which is a packed structure that provides a large wetted surface area to induce intimate contact between the gas and the scrubbing liquid. The contaminant is absorbed into or reacted with the scrubbing liquid.
Wet scrubbers can be divided into two basic categories: vertical and horizontal airflow models. In one common type of vertical wet scrubber, a reaction chamber contains layers of variously-shaped packing material, such as Rashig rings, spiral rings, or Berl saddles, that provide a large surface area for liquid-particle contact. The packing is held in place by open mesh retainers and supported by a plate near the bottom of the scrubber. Scrubbing liquid is evenly introduced above the packing and flows down through the bed. Liquid coats the packing and establishes a thin film. The pollutant to be absorbed is soluble in the fluid. In vertical designs, the gas stream flows up the chamber (countercurrent to the liquid).
A horizontal wet scrubber works on essentially the same principles. A scrubber vessel defines a reaction chamber. Dividers within the reaction chamber define a serpentine path from the inlet to the outlet. This serpentine path increases the residence time of a gaseous stream in the reaction chamber. The reaction chamber is typically filled with a packing material. The packing material is normally a proprietary, loose fill, random packing designed to encourage dispersion of the liquid flow without tracking, to provide maximum contact area for a “mass transfer” interaction, and to offer minimal back pressure to the gas flow. The reaction chamber has chemical spray nozzles that distribute the caustic solution over the tower packing media. The scrubber vessel is located on the top of a fluid storage tank, which is an integral part of the system. A screen separates the reaction chamber from the fluid storage tank and supports the packing material, while permitting the caustic solution to drain into the storage tank. A recirculation pump drains the caustic solution, e.g. sodium hydroxide (NaOH), from the bottom of the fluid storage tank and circulates it through the reaction chamber. A fan draws contaminated gas through the system such that the gas stream flows crosscurrent to the liquid. The gas stream passes through the reaction chamber, is absorbed by reacting with the caustic solution, and is exhausted through a stack. A demister is fitted at the top of the tower to prevent entrainment of droplets of the scrubbing liquor into the extraction system or stack.
One use for a wet scrubber is to remove chlorine gas that might leak from a storage cylinder. Facilities storing hazardous quantities of chlorine or sulfur dioxide must invest in emergency standby equipment to prevent accidental chemical releases. The Environmental Protection Agency's (EPA's) Risk Management Program for Chemical Accident Release Prevention “requires regulated facilities to develop and implement appropriate risk management programs to minimize the frequency and severity of chemical plant accidents.” In addition, “a performance-based approach towards compliance with the risk management program rule is required.”
The Uniform Fire Code, Article 80, states that the full contents of the single largest storage container of chlorine must be mitigated in thirty minutes. If a toxic gas release were to occur from a one-ton cylinder of chlorine, the laws of thermodynamics suggest that approximately 400 lbs of liquid chlorine would flash into vapor, and the remaining contents of the chlorine cylinder would spill out as a liquid at its boiling point.
Chlorine cylinders are stored in a separately enclosed storage room, which is equipped with a chlorine detector. During normal automatic system operation, when a chlorine leak or spill occurs, the chlorine detector activates the system in two steps. The pump is activated first to permit proper and complete wetting of the packing material with the caustic solution. Next, the exhaust fan is activated. The exhaust fan is placed downstream of the scrubber. This placement of the fan allows the complete system, including the storage room ducting and scrubber, to remain under negative pressure until all chlorine is removed. Thus any leaks draw air into the system, rather than leaking out contaminated gas. An adjustable time delay is provided to delay the start of the exhaust fan and subsequent movement of the air through the scrubber. This feature allows all of the chlorine laden air to fully react and substantially eliminates the possibility of any chlorine gas passing through the scrubber and into the atmosphere.
Wet scrubbers suffer a number of disadvantages. The NaOH used in the scrubber is highly caustic. Thus proper construction materials and methods are very important to prevent leaks from the tank. A recirculation pump and associated plumbing are required to constantly pump the scrubbing fluid from the tank and to spray it over the packing material in the scrubbing vessel. The pump requires energy to operate, and the distribution pipes present another source of possible corrosion and leaks.
For these and other reasons, a dry scrubber is often a preferred solution. Dry scrubbers pass the gaseous stream through a granular solid catalyst. The pollutants in the gaseous stream react with the catalyst and are neutralized. No corrosive liquids are employed, and hence the danger of a leak is eliminated. In addition, no recirculation pump is required, eliminating a source of energy usage and possible mechanical problems.
It would therefore be advantageous in many applications to convert an existing wet scrubber to a dry scrubber, preferably in a way that maximizes re-use of existing components. This is not a straightforward modification, however. The granular dry catalyst is much more dense that a corresponding volume of wet packing material. Thus if one were simply to remove the packing material and replace it with a corresponding volume of dry catalyst, the pressure drop through the serpentine path would make it impossible to meet regulation requirements. In the example given above of chlorine cylinders stored in a separately enclosed storage room, there are requirements that the airflow be extracted from the storage room at a rate of no less than 2,238 ft3/min. (the rate at which chlorine gas escapes from a ruptured cylinder). On the other hand, if the dividers are removed so as to reduce the pressure drop, then the residence time of the gaseous stream within the reaction chamber is not sufficient to neutralize the contaminants. Thus simply substituting a dry catalyst for the packing material of a wet scrubber will not produce satisfactory results.
Thus there is a need for a method for converting a wet scrubber to a dry scrubber.
There is a further need for a method for converting a wet scrubber to a dry scrubber in a manner that maximizes the re-use of existing equipment.